This invention relates to methods and apparatus for providing screened plates used in printing half-tone graphics images and, in particular, relates to electronic screening of scaled density information representing a multi-color continuous-tone graphics image with screen matrix information data to produce binary element information data. The binary element information data thereafter is used to image electrophotographic members and produce thereon color separation screened representations of the original continuous tone image, each screened representation being at the correct angle.
A continuous-tone image is any painting, scene or photograph in which there is a broad range of tones or gradation of tones. In a black and white photograph, these tones may be represented by varying amounts of black-looking, developed silver. The more silver in certain areas, the darker or blacker the tone. In color photographs the tones are represented in the same way by varying amounts of developed dyes.
In letter press and offset lithography, continuous tones cannot be printed directly by varying the amounts of ink. A printing press can print only a solid area of ink while leaving other areas unprinted. Half-tone printing reproduction of continuous-tone images was developed to overcome this limitation of a printing press.
Half-tone printing uses the poor resolution of the human eye to reproduce the continuous-tone image with an array of dots. In manual half-tone printing these dots are regularly spaced apart and are of variable area. Portions of the image that are dark are known as the shadows and have large area dots; portions that are light are known as highlights and have small area dots. The human eye fails to recognize individual ones of the dots due to its poor resolution and tends to integrate the dots together into apparent continuous tones.
A single dot array for printing one color or black is produced by photographing the continuous-tone image through an orthogonal screen of lines. Shadows of the original image produce dots of large area in the photographic positive while highlights produce dots of small area in the photographic positive. The photographic positive then may be used as the printing plate to print certain solid areas with ink while leaving other areas unprinted.
This is the type of graphics reproduction often used in newspapers. Increasing the number of lines per inch produces a sharper printed image. Typically screen rulings or frequencies are from 65-300 lines per inch with the finer rulings being used for higher quality reproduction, and 150 lines per inch typically being used for offset lithography.
Color continuous-tone images are reproduced with a plurality of dot arrays using the three color theory of white light. Under that theory white light will be perceived by the human brain from the addition of red, green and blue colors, the additive primaries. By photographing the continuous-tone image through filters of the additive primaries and the half-tone screen, three separate arrays of dots are formed. These dot arrays then are printed using the subtractive primary colors of yellow, cyan and magenta to reproduce the color of the original image. An array of black dots also is printed to increase the quality of the printed image.
Better quality color graphics printing is obtained by increasing the number of different colors that are printed. This overcomes insufficiencies in the dyes available for both the inks and filters. Of course, the array of dots for each printed color is formed by photographing the original through a related color filter and a half-tone screen.
The arrays of dots are printed with the dots arranged at angles different from one another. This is to avoid over-printing each color dot on the previously printed dot or dots and to avoid Moire patterning. Moire patterning results when screens of equal frequency lines are slightly offset from one another and produces an undesirable effect in the printed image. These angles are obtained not by rotating the dot arrays after they are photographed through the screens, but by rotating the half-tone screen to different screen angles during the formation of the dot arrays on the photographic negative.
Moire patterning is minimal when the screen angles are about 30.degree. apart from one another. The angles typically used for the four screens are: black 45.degree., Magenta 75.degree.; yellow 90.degree. and cyan 105.degree. from a vertical reference. The screens are formed of orthogonal lines and thus the angles may also be related to a horizontal reference and further may be described as: black +45.degree., magenta +15.degree., yellow 0.degree. and cyan -15.degree..
Manual half tone processing is labor intensive, time consuming and expensive. It requires considerable skill and much capital equipment to produce large quantities of printing plates for such as the printing of periodicals, magazines, books and other widely circulated printed matter.
Manual half-tone processing recently has been implemented electronically. Arrays of orthogonally aligned dots are formed in which the dots are regularly spaced from one another and have equal areas, but are much smaller than the dots typically used in manual half-tone printing. Shadows are represented by forming several dots close to one another while highlights are represented by forming few dots far from one another.
In electronic half-tone processing the original continuous-tone image is scanned with a light sensitive instrument to determine the scaled density of sequential adjacent incremental areas. The output of such scanning apparatus is a stream of multibit digital words with the value of each word representing the scaled density of an individual incremental area and the entirety of the words representing the density of the original image.
The stream of digital words is operated on by a screening circuit. The output of the screening circuit is a series of binary signals indicating the formation of binary elements on an image member. The image member is capable of having binary elements formed thereon in orthogonally arranged rows and columns of incremental areas by such as radiant energy means such as a laser. The binary signals indicate whether an element will or will not be formed in the incremental areas and control whether or not the radiant energy will impinge on the image member.
Each density value word from the scanner generally will determine which ones of a group of binary elements will be formed, the groupings of elements being generally referred to as picture elements or pixels. Thus, one density value digital word from the scanner apparatus will control the formation of binary elements in one image pixel.
After the image member is completely imaged, the member is operated on to form a printing plate, one image member forming a printing plate for each color.
In color electronic half-tone processing the original image is scanned through different color filters as in manual processing. Each such scan produces a set of scaled density data corresponding to the original image, less the filtered color. But electronically screening the scaled density data has been a problem.
The problem is introducing the screen frequency and angle information into the scaled density data of each color so that the imaged electronic dots are properly overlapped, if desired, and are properly registered one to another to avoid Moire patterning. The data from the scanning apparatus is only scaled density information with corresponding data in each color separation being obtained simultaneously from one incremental area. The scanning apparatus usually does not scan at different locations for each color. Essentially, the data from the scanning apparatus is devoid of all screen information. In manual half-tone processing screen angle information is provided by rotating the screen to different angles and overlap and moire patterning are avoided by proper registration of the photographic and printing plates. Screen frequency information is obtained by the number of lines per inch in the screen itself. Electronic half-tone processing somehow must provide this angle and frequency information.
Introducing the screen angle information into the data from the scanner by scanning at different screen angles is not desired. The scaled density data for each of the several colors are obtained simultaneously from one incremental area by scanning the area with white light and separating the different color densities with color filters. Scanning at different locations and angles for each color separation unacceptably multiplies the scanning time by the number of colors to be scanned.
The prior art does not solve this problem. U.S. Pat. No. 3,922,484 to Keller performs electronic screening without regard to overlap or Moire patterning. The density data for each color separation is imaged according to a predetermined format of density representing printing patterns and the printing patterns are printed on the receptor with whatever super impositioning that occurs.
U.S. Pat. No. 4,012,584 to Gascoigne introduces the screen matrix information including the screen angle into the density data through the use of "1" shifting shift registers. The density data from the scanning unit are acted on by a color computation unit that controls the length of trains of binary "1" signals introduced into the opposite ends of pairs of shift registers. The shift registers then are stepped in opposite directions and whenever corresponding locations of the pair of shift registers contain binary "1" signals, a binary element is formed by a light source element. Varying screen angles are obtained by stepping the shift registers of each pair at different frequencies.
U.S. Pat. No. 4,051,536 to Roetling performs the screening function by summing the scanned density information from several incremental areas corresponding to the screen with the values of the half-tone screen function. In another channel, the same scanned density information from the several areas is averaged and the average determines the percent of dots to be formed in that area. The summation then is thresholded until the determined percent of dots is formed. No color half-tone screening at different angles is disclosed.
U.S. Pat. No. 4,196,453 to Warren performs the screening function by storing a line-at-a-time the scaled density information data in buffers and operating on the buffered data in blocks of twenty lines by twenty pixels. The scaled density information datum for each pixel on each line then is compared to a screen matrix value stored in a memory device and results in a binary output from the comparator. The overall density value of each block is ascertained to select between available screens. The binary output from the comparator then is used to provide an image on a xerographic member. No disclosure is made of imaging at different screen angles and no color processing is disclosed.
Further, it would be desirable to introduce the screen frequency and angle information into the scaled density information for varying frequencies and angles. The electronically screened frequencies must at least equal the currently used manually produced screen frequencies of 65-300 lines per inch and preferably should exceed this range. The angles provided electronically must at least be those currently used and preferably should be any angle desired. Selection of the desired screen frequency and angle must occur simply and with the use of unskilled labor to improve over the cost of manual half-tone processing.
The time required to perform the electronic screening also is important. Processing an original image electronically from scanning to ready-to-print plates must be at least as fast as manual half-tone processing. Preferably, electronic screening and imaging on an electrophotographic member should occur as fast as scanning, and should be fast enough to accept scanned data directly from long distance data links to realize real-time processing economics.